Invulnerability Hunter

The Invulnerability Hunter is a boss monster found in Doom 3: Resurrection of Evil, the third and final Hell Hunter the player will encounter. Similar to the Berserk Hunter, the Invulnerability Hunter is noticeably larger than a Hell knight, and it has an extremely muscular physique with a blue electric aura surrounding it. It occupies the Energy Core area of Phobos Labs - Sector 2: Molecular Research, where it can draw energy from the power coils to make itself invulnerable. One of the power coils overloads and is destroyed during the cutscene of the Hunter's introduction, interrupting its attempt to become invulnerable - this is a clue about how to defeat the monster. The Invulnerability Hunter's attacks include melee strikes, hurling an electrified plasma orb, and stomping the ground to unleash an electric shockwave. This wave cannot hit the player if they are at a higher elevation, so there are safe areas in the room that can be used to escape it.

When the Invulnerability Hunter has been defeated, the Artifact will absorb the Hunter's Invulnerability power. Once invoked, this power protects the player from almost all enemy attacks, although certain environmental effects can still harm them.

In the original game, the arena in which the Hunter is fought includes a hidden underground area, accessed by lifts on either side of the room. The underground area contains several Artifact charges, a health station and two ammo belts. However, the player is not safe in the underground area - the Hunter can harm them with its shockwave stomp attack. The underground area is removed from the game in the BFG Edition; there are more supplies and Artifact charges on the upper steps instead.

Combat characteristics
The Invulnerability Hunter's behaviour can be divided into two phases - its normal phase and its invulnerable phase. The battle begins with the Hunter in its normal phase. In its normal phase, the Hunter will prowl the centre of the room, throwing its plasma projectiles at the player, delivering its shockwave stomp and melee strikes if the player gets too close. The room has steps around its border, and the Hunter will not attempt to climb these steps. While in its normal phase, the Hunter can be harmed by the player's weaponry. After taking a certain amount of damage, the Hunter will enter its invulnerable phase - the glow around the Hunter will intensify and the room will darken to indicate that this is happening.

In its invulnerable phase, the Hunter will draw energy from the power coils in the room, rendering it immune to the player's weaponry. While invulnerable, it will remain still and continue to throw its plasma projectiles at the player. However, one of the power coils can now be exposed by the player - this is done by pressing the green button on its housing (only one specific power coil can be opened at a time; the other buttons will remain red). This button exposes the power coil which can then be destroyed by the player's weaponry. Doing this will break the Hunter out of its invulnerable phase and return it to its normal phase.

The player has infinite stamina during this battle, just the same as when the player is in Hell or fighting the cyberdemon near the Hell Hole in the caverns.

Tactical analysis
During the Hunter's normal phase it is susceptible to damage for a period, during which the Artifact can be used in order to maximize damage dealt and easily dodge the Hunter's return fire. It is wise to remain on the upper steps, where the Hunter's shockwave stomp and melee attacks are no threat - only its thrown plasma projectiles can harm the player. The chaingun is arguably the best weapon to use due to its excellent damage output and ease of use against a large target, although the Ionized Plasma Levitator is also a good choice - if the player remains upon the upper steps, then only the Hunter's thrown projectiles can harm them, which can be seized by the grabber and launched back. The grabber is therefore a safer way to fight the Hunter (and saves ammunition), although the chaingun does the damage more quickly. It requires five hits from the Hunter's own projectiles to cause enough damage for it to proceed to its invulnerable phase, which the chaingun can achieve in quicker fashion.

After dealing a certain amount of damage to the Hunter, the room will darken and the energy aura surrounding it will intensify, indicating that it is entering its invulnerable phase. Once its invulnerable phase begins, there is no point shooting the Hunter. However, one of the power coils in the room can now be exposed, indicated by the button on its housing turning green. The player must press this button and then shoot the power coil once it is exposed. Destroying the coil can harm you if you are standing close to it when it explodes, so it is best to press the button and then back away to shoot the coil at range with a weapon such as the rocket launcher - three direct rocket hits can destroy the coil in the original game or two rockets in the BFG Edition, but the Hunter will still throw its plasma projectiles at the player while this is happening, so the player must take care to dodge while pressing the button and shooting the coil. It is not a bad idea to use the the Artifact once the Hunter has entered its invulnerable phase, allowing the player to easily dodge the projectiles thanks to Helltime so the button can be pressed and the coil destroyed safely. After the coil has been destroyed, the Hunter will soon re-enter its normal phase.

The overall process of damaging the Hunter during its normal phase and then breaking it out of its invulnerable phase will need to be repeated three times. After all the coils have been destroyed, the Hunter will have a small amount of health remaining until it is completely defeated.

While it may be tempting to try to use the Berserk power of the Artifact to conserve ammo by punching the Hunter with the player's fists, the player will find that the electric aura of the Hunter protects it somewhat against melee attacks, damaging the player and hurling them backward, so this is only worth considering on Nightmare difficulty (see below).

As described above, the player must expose and destroy the three power coils, and only one of them can be opened each time the Hunter enters its invulnerable phase. The three coils are to be opened in a specific order: if one were to look at the room from the open doorway, then the first coil to be exposed is the one on the near left, then the one on the near right, and finally the one on the far left. If the player remembers this specific order then they can aim to be standing close to the correct coil for when it will need to be opened.

Nightmare difficulty
When playing on Nightmare difficulty, the player's Artifact is already empowered with the invulnerability effect which opens up a new strategy. The Hunter is naturally resistant to melee attacks, its electric aura damaging you and hurling you backward if you try to engage it in melee, while taking less damage from your attacks - even if you are Berserk. However if you are invulnerable as well, then you can safely deliver Berserk punches to the Hunter. Therefore, on Nightmare difficulty it is a good tactic to activate the Artifact and repeatedly punch the Hunter - its electric aura will occasionally hurl you backward, but you won't take damage in the process. It takes around ten berserk punches to make the Hunter enter its invulnerable phase, which can be done safely and without expending ammunition.

Trivia
One of the textures used with alpha blending to create the electric effect around the Invulnerability Hunter is present in the base files for Doom 3. This was initially thought to be an unused monster by fans on the Doomworld forums until it was properly identified.